Method of making nickel-chromium-beryllium alloy

ABSTRACT

This invention relates to a method of making nickel-chromiumberyllium alloy having a microstructure which provides obstacles to plastic flow in the nickel rich matrix phase so that high strength properties are obtained at both room and elevated temperatures. The alloy may also contain other additives including aluminum and/or titanium together with hardeners including molybdenum and/or tungsten, with or without carbon. A method of processing the alloy comprises melting the alloying ingredients in a crucible while avoiding the presence of oxygen; raising the temperature to 1,450* to 1,550*C; casting the alloy in a mold. The alloy may be remelted and cast in an apparatus to effect abstraction of heat from the casting from one direction only to obtain a directionally orientated microstructure.

United States Patent [191 Griffiths et al.

[451 Aug. 21, 1973 METHOD OF MAKING NICKEL-CHROMIUM-BERYLLIUM ALLOY [75] Inventors: Leonard B.'Griffiths; Yuan-Shou Shen, both of North Reading, Mass.

[73] Assignee: P. R. Mallory & Co., Inc.,

Indianapolis, Ind.

[22] Filed: Apr. 21, 1971 [21] Appl. No.: 136,229

Related U.S. Application m [60] Division of Ser. No. 832,335, June 11, 1969, Pat. No. 3,704,182, which is a continuation-in-part of Ser. No. 793,814, Jan. 24, 1969, abandoned.

[52] U.S. Cl. ..-148/162 [51] Int. Cl. C22f 1/10 [58] Field of Search 148/162, 3, 32, 32.5; 75/171 [56] References Cited UNITED STATES PATENTS 2,469,718 5/1949 Edlund et al. 75/171 3,464,817 9/1969 Griffiths 75/171 3,260,505 7/1966 Ver Snyder 75/171 8/1937 Touceda ..75/171 3/1939 Touceda 75/171 Primary ExaminerRichard 0. Dean Attorney-Charles W. Hoffmann [57] ABSTRACT This invention relates to a method of making nickel-chromium-beryllium alloy having a microstruc ture which provides obstacles to plastic flow in the nickel rich matrix phase so that high strength properties are obtained at both room and elevated temperatures. The alloy may also contain other additives including aluminum and/or titanium together with hardeners including molybdenum and/or tungsten, with or without carbon. A method of processing the alloy comprises melting the alloying ingredients in a crucible while avoiding the presence of oxygen; raising the temperature to l,450 to 1,550C; casting the alloy in a mold. The alloy may be remelted and cast in an apparatus to effect abstraction of heat from the casting from one direction only to obtain a directionally orientated microstructure.

8 Claims, 7 Drawing Figures sum 1 -ur 4 PATENIEB MICZI B15 sum 3 or 4 u I 2 V "nun" I I'nnI' IVIIII Ill lllflllnf /f \l O I 4 3 gggRoslom IN AlR I00 HOURS STRESS psix 10- Pmmimuhza ms 3.753.800

SHEET 8 0F 4 LARSONM|LLER PLOTS OF STRESS-RUPTURE DATA "P" T (Logt+2O)xlO" This isa'division of application Ser. No. 832,335, filed June 11, 1969, now U.S. Pat. No. 3,704,182 which in turn is a continuation-impart of application Ser. No. 793,814, filed Jan. 24, 1969, now abandoned.

Alloys containing nickel-chromium and beryllium have been known for some time. For example, such alloys are disclosed in British Pat. No. 404,012 (for use in surgical sewing needles and dental instruments) and US. Pat. Nos. 2,089,587, (for dentures) and 2,150,205 (for engine valves).

However, the advantage of using these alloys in specific proportions resulting in a specific microstructure for both room temperature and high temperature applications requiring high mechanical properties has not previously been recognized.

It is, therefore, an object of the present invention to provide a Ni-Cr-Be alloy having high strength at elevated temperatures.

It is another objectof the present invention to provide an alloy which is resistant to oxidation at elevated temperatures.

It is another object of the present invention to provide an alloy having high stress-to-rupture values at elevated temperature.

It is another object of the present invention to provide an alloy having good mechanical properties at room temperature.

It is another object of the present invention to provide an alloy having a microstructure which is resistant to plastic flow at both room and elevated temperatures.

It is another object of the present invention to provide a high temperature alloy which avoids the use of carbides for strengthening.

It is still another object of the present invention to provide a method for processing a. high temperature alloy containing nickel-chromium-beryllium in order to obtain a microstructure which has resistance to plastic flow at room and elevated temperatures.

It is another object of the present invention to provide a process for treating an alloy in order to obtain uniaxial as-cast structure. 1

Other objects will be apparent from the following description and drawings in which:

FIG. 1 is a view of an exemplary microstructure of the alloy of the present invention at 2,700 magnifications.

FIG. 2 is a ternary Ni-Cr-Be diagram showing the composition of the eutectic alloy utilized in obtaining the desired microstructure according to the present invention.

FIG. 3 is a plot of weight gain (oxidation) as a function of time, at various temperatures, for the alloy of the present invention and other representative alloys of the prior art.

FIG. 4 is a schematic diagram of apparatus which may be used in casting the alloy of the present invention.

FIG. 5 is a view of the as-cast ingot showing the microscopic directionality of the casting at 20 magnifications.

FIG. 6 is a view of the microstructure of the alloy of the present invention in the directionally cast condition at 1,300 magnifications.

FIG. 7 is a Larsen-Miller plot of the stress-rupture data obtained in Example 11.

As can be seen from FIG. 1, the microstructure of the nickel-chromium-beryllium alloy of the present invention is composed of a nickel rich solid solution matrix 10 having chromium dissolved therein in solid solution and nickel-beryllide rods (NiBe) 11, in which chromium is also dissolved, interspersed within said matrix.

While the structure is actually the result of a three phase equilibrium transformation, it is useful to consider it as a binary eutectic in which:

(1 Liquid Ni NiBe The reaction occurs at the base of a liquidus valley bounded by the two-phase fields, liquid Ni(Cr) and liquid Ni(Cr)Be, within the Ni-Cr-Be constitutional system. As can be seen from FIG. 2, the composition at which this reaction takes place is within the temperature range of l,l to 1,275C. The nickel content of the alloy of the present invention should be about 54 to 72.5 atomic percent. The chromium content should be about 32.5 to 2.5 atomic percent and the beryllium content about 14.5 to 25.5 atomic percent. Furthermore, the composition should be :such that upon freezing no two-phase field is intersected, but rather the reaction proceeds at all times in accordance with the three phase reaction given in. equation (1). This occurs at compositions substantially as shown by the solid line in FIG. 2. A variance of about :2 atomic percent of the Ni-Be ratio on either side of the solid line in FIG. 2 may also be used and the desired microstructure will be obtained. Since freezing occurs essentially within the liquidus valley, the temperature dependence of which is only slight, a substantially uniform microstructure is obtained throughout the ingot, in contrast to alloys having a long freezing range.

The presence of the Ni(Cr)Be :rods 11 is to be con trasted with the nickel-beryllium binary system wherein the NiBe phase is plate-like. It has been found according to the present invention that the addition of chromium results in the desired rod-shaped beryllide phase.

Generally, in two phase structures of this type, one of the phases predominates at the grain boundaries which results in either brittleness or low strength; however, the beryllide rods extend completely to the colony boundaries in the alloy of the present invention, as shown in FIG. 6. This phenomenon is very important. The beryllide rods provide obstacles to plastic flow in the nickel rich phase. This feature can be seen in FIGS. 1, and 6; showing slip lines 12 are prevented from propogation'by the hard beryllide phase 11. This phenomenon yields a relatively high rate of work hardening. Spacing between rods within a colony is usually within the range of about 2 to 20 u, and preferably about 4 to 8 u. This is computed on the basis of the mean free path distance.

The chromium, in addition to rendering the beryllide phase rodlike, confers some solid solution strengthening both on the nickel and on the beryllide phase. The chromium also provides oxidation resistance. This can be seen from FIG. 3. It is apparent from FIG. 3 that at 1800F there is less weight gain (a measure of oxidation) with the alloy of the present invention than with the Inconel (10 Cr, 15 Co, 5 Ti,.5.5 Al, 3 Mo, 0.18 C, Bal. Ni) and Rene-Y (Cr 22, Mo 9, Fe 18, W 1 max.,

Co 2 max., Mn 1.0, Si 1.0 max., 0.l5 C, 0.15 La, Bal. Ni) prior art alloys.

Oxygen and nitrogen should each be below 0.5 percent by weight respectively. Carbon should be below 0.02 percent by weight unless carbides are to be formed as described hereinafter.

The alloy of the present invention may be prepared by melting together the constituents preferably in elemental form in a refractory crucible made, for example, of MgO, M 0 or BeO in the absence of oxygen. If desired, the melting may be carried out in an inert atmosphere such as argon or helium. It is also often desirable to utilize a reduced pressure or partial vacuum atmosphere. The melting temperature should be between about 1,450 to l,550C. in order to insure that ample fluidity for subsequent casting is obtained.

The mold to be used in casting the alloy of the present invention may be either of the permanent mold type, for example, made of copper or steel or graphite molds may be utilized. If graphite is utilized, it is preferred to use a beryllium oxide wash in the mold. The casting cooling rate for the casting operation is not critical.

After casting, the ingots are preferably remelted and subsequently frozen uniaxially in order to obtain the best properties. For example, one apparatus which may be used for the remelting operation is shown in FIG. 4. The holding screws and 21 hold plates 22 and 23 in place by means of spring fasteners 24. A shielding plate 25 is placed upon the top of holding plate 22 and a chillplate 26 is placed thereon. The chillplate has a water inlet 27 and a water outlet 28 associated therewith which pass through plates 23 and 25 for circulating a cooling fluid therethrough. A starting stub 29 is mounted on the chill plate.

The ingot to be melted 31 is placed in a refractory crucible which may be made of MgO, Al O or BeO or other refractory oxide materials. Inorder to effect melting an RF coil 50 is energized to provide sufficient heat to reach the melting point temperature range given above.

The lowering mechanism comprises a motor 60 mounted on the base 61. The motor is connected to a wire rope or chain 62, which by means of idle wheels 63 and 64, controls the decent of argon adaptor 70 by means of hook 65 in engagement with the flange 71 of the argon adaptor. The adaptor has an inlet 73 and an outlet 74 for circulation of argon gas. The inlet conduit 73 is in communication with the crucible 73 to provide an argon atmosphere during the melting operation which takes place in the vicinity of coil 50. The argon then passes out of the crucible and through conduit 75 and 74.

The argon adaptor is affixed to the crucible 30 by conventional fasteners (not shown) so that the adaptor, the crucible and the entire structure excepting coil 50 are lowered upon operation of motor 60, causing the ingot to be melted to move. through coil 50. The amount of movement is limited, however, by the space between the coil and shielding sheet 40.

Initially the motor 60 is utilized to place the meeting point of the starting stub and the ingot to be melted within RF coil 50. RF coil is then energized sufficiently to melt a cross section of the ingot. The ingot is then gradually lowered as freezing occurs from the chill plate 26 upward under the action of the naturally occurring axial temperature gradient from the chill plate upward. For example, the ingot may be lowered at the rate of 1 inch to 2 inches per hour. The casting operation is stopped when the coil reaches a point near shielding sheet 40 or when all of the ingot has been melted.

It will be apparent that this arrangement results in a directionally frozen ingot as shown in FIG. 5. However, it will be apparent to those skilled in the art that other casting apparatus which provides for directional freezing can also be utilized. For commercial scale operation, continuous casting equipment in which the ingot is continuously cooled by application of a cooling medium to the already cast portion of the ingot, could be utilized.

The microstructure of the alloy of the present invention in the directionally frozen condition is shown in FIG. 6 and the nickel-matrix 10 again surrounds the Ni(Cr)Be rods 11. The microstructure is also composed of two phase colonies which are greatly elongated in the direction of casting growth; in other words, parallel to the growth axis as shown in FIG. 5. The beryllide phase 21 is uninfluenced by growth conditions, and the axis of the rods varies from colony to colony. Note the rods in colonies 23 and 24. There is a preferred direction however within each colony. The elongated colonies result in markedly reduced transverse colony boundary area.

In addition to the amounts of Ni, Cr and Be previously stated, a metal selected from the group consisting of aluminum, titanium and mixtures thereof is preferably added in the range of from about each I to 5 atomic percent, preferably 2 to 4 atomic percent, each. Preferably aluminum alone, or mixtures of aluminum and titanium are used. The aluminum and/or titanium present reacts with nickel to form the compounds such as Ni Al and Ni Ti. These compounds may be distributed throughout the nickel-rich phase by first providing a solution treatment at a temperature of about l,950 to 2,150F preferably about 2,000 to 2, 1 50F for a period of 2 to hours, preferably 4 to 50 hours. This heating step is followed by ageing at a temperature of from about l,200 to l,500F, preferably l,200 to l,400F for about 2 to 30 hours, preferably 4 to 16 hours. This results in the Ni Al and/or (Ti, Al)Ni precipitating from solid solution in the form of the a phase well known in the art.

It is also preferred to add one or more hardening elements selected from the group consisting of Mo, W, Ta, Nb, Va, Zr, B, Mn, Fe, Co and mixtures thereof, in an amount of from about 2 to 10 atomic percent total. Preferably Mo and W are used in-the range of about 2 to 2.5 atomic percent Mo and from about I to 1.5 atomic percent W. The hardening addition will affect an increase in strength by solid solution hardening of the nickel-rich phase.

The hardening elements will also form carbides if free carbon is present. While considerable strengthening can be obtained by carbide formation by including carbon in the range 0.06 to 0.12 percent by weight, according to the preferred embodiment of the invention the carbon content is restricted to not more than 0.02 percent by weight so that carbide formation is substantially avoided. During service at elevated temperature the carbides have a tendency to coarsen, causing embrittlement.

As to the properties of the alloy of the present inven tion, Table I shows thatthe alloy of the present invention is superior to representative alloys of the prior art from a standpoint of modulus of elasticity and specific tal constituents in the crucible. The crucible was placed in a furnace and heated to above the melting point of the resulting. alloy in an argon atomsphere. The alloys were poured at l,450C into steel molds.

Alloys having the composition given in Tablell were melted in a refractory crucible, by placing the elemenmodulus (modulus of elasticity divided by density). 5 The resulting castings were 7 inches long by 1% inches in diameter.

Those specimens which were directionally cast were TABLE I remelted and directionally cast in the manner described above in regard to FIG. 4. Casting rates were ggg g fglg l 1 to 2 inches per hour.

. pw 4 3 5 The specimens which were heat treated were solution 8 g2 12 5 2 Ta treated at l,lC for 4 hours and then air quenched llasiela x 100 to 760C and held for 16 hours. 2 C 20 e. 9 C. Ni B 30 m4 Tensile specimens were than prepared and standard I 15 tensile tests were carried out at the temperatures given 13 Cr. 6.0 Al, 4.5 Mg, Ta Ch 2, Ti .6, Co in Table III. Room temperature was 68 to 75F.

a'g fgfi' Fe The tensile properites are reported in Table III. In the nm 30 m tableC. C. indicates the specimens were convention- 1950, 555 S M 4-5 l ally cast; D. C. indicates the specimens were directionl 129 20 ally cast and HT indicates that the specimens were heat Alloy of Present Invention treated prior to testing.

TABLE III Mechanical Properties of N i-Cr-Be Alloys Specific mechanical properties T t Mechanical properties X1000 p.s.i. X1000 in.

ES Alloy Condi' tenlp., Prop. 0. 2% Elollg., 0.2% No. tion F. limit Y.S. U.'I.S. percent Y.S. U.T.S.

Roon1 55.5 78.2 132.4 s 290 490 C c -51.0 78.b' 132.5 -2n4 -491 16. 299 334 1 -38? 252 -52 110. 56.7 53.0 48.3 D.C. 94.2 H.'I. 95.2 2 D.C. 83.6

H.'l. D.C. D.C. H.'I. 13.0. 3 H.l".

H31. H 0.0. HIP. DIG. B31. 4 no. IIJI. 1;.(1. in.

EXAMPLE I EXAMPLE II Tensile Tests 55 Stress-Rupture Tests The alloy compositions chosen for testing are given Test specimens of the alloys listed in Table IV were i T bl II, prepared 2% inches long with a gage length of threequarters inch and a diameter of 0.187 to 0.189 inches TABLE n in accordance with ASTM E-8. The specimens were then placed in a standard ele- ALLOY COMPOSITION ATOMIC vated temperature tensile testing machine. Alloy No Ni Cr Be M Ma w C The specimens were stabilized for 15 to 30 minutes 1 65.0 15-0 2 at temperature and then the stresses shown in Table IV 3 gg-g :g-g 5-8 5-; 0 45 were applied to the alloys indicated in Table IV at the t 5 1.0 1910 [6:0 4:0 2:5 1.5 temperatures indicated in Table: IV in an air atmosphere.

The specimens failed after the time indicated in Table IV.

TABLE IV STRESS RUPTURE PROPERTIES OF Ni-Cr-Be ALLOYS Al- Conloy di- Stress Rupture Time (hr.) No. tion psi 1200"! i300"? 1400'] i500? I C. C. 45,000 98.1 20.35 l.45 0.2

C. C. 35,000 I744 2.30 0.25 2 D. C.

H. T. 35.000 18.40 3.0 3 D. C.

H. T. 45,000 90.3 B 0.8

H. T. 35,000 1280.75 33.95 3.4 4 D. C.

H. T. 45,000 I l4.0

The data listed in Table IV is shown in a Larsen- Miller plot (described in Metals Handbook, 1961 Ed. pp. 472-473).

What is claimed is:

l. A method of preparing a Ni-Cr-Be type material including the steps of preparing in a non-oxidizing atmosphere a melt consisting essentially of about 54 to about 72.5 atomic percent Ni, about 32.5 to 2.5 atomic percent Cr and about 14.5 to about 25.5 atomic percent Be in amounts that solidification proceeds by the three phase reaction Liquid (-2 Ni NiBe, and solidifying the melt obtaining a microstructure comprising a rod-like phase containing Ni(Cr)Be dispersed in a Ni rich matrix.

2. The method of claim 1 including the further step of treating the microstructure to elongate colonies of the rod-like Ni(Cr)Be phase uniaxially.

3. The method of claim 2, wherein the step of treating the microstructure includes the step of abstracting heat from the melt from one direction obtaining elongation of colonies of the rod-like Ni(Cr)Be phase uniaxially.

4. The method of claim 3, including the further steps of, prior to the step of abstracting heat from the melt, cooling the melt of Ni-Cr-Be type material and remelting the Ni-Cr-Be type material.

5. The method of claim 1, including the further step of adding to the melt up to about 5 atomic percent of a metal selected from the group consisting of Al, Ti or mixtures thereof.

6. The method of claim 1, including a further step of adding to the melt up to about 10 atomic percent of a metal selected from the, group consisting of Mo, W, Nb, Zr, B, Va, Ta, Mn, Fe, Co or mixtures thereof.

7. The method of claim 1, including the further step of adding to the melt up to about 5 atomic percent of a metal selected from the group consisting of Al, Ti or mixures thereof, and adding to the melt up to about 10 atomic percent of a metal selected from the group consisting of Mo, W, Ta, Nb, Va, Zr, B, Mn, Fe, Co or mixtures thereof.

8. The method of claim 5, including the further steps of heating the Ni-Cr-Be type material containing the additive metal or mixtures of additive metals for a sufficient time to dissolve additive metal or mixtures of additive metals in solid solution, and strengthening the NiCr-Be type material by holding the material below the solution temperature for a sufficient time to effect strengthening. 

2. The method of claim 1 including the further step of treating the microstructure to elongate colonies of the rod-like Ni(Cr)Be phase uniaxially.
 3. The method of claim 2, wherein the step of treating the microstructure includes the step of abstracting heat from the melt from one direction obtaining elongation of colonies of the rod-like Ni(Cr)Be phase uniaxially.
 4. The method of claim 3, including the further steps of, prior to the step of abstracting heat from the melt, cooling the melt of Ni-Cr-Be type material and remelting the Ni-Cr-Be type material.
 5. The method of claim 1, including the further step of adding to the melt up to about 5 atomic percent of a metal selected from the group consisting of Al, Ti or mixtures thereof.
 6. The method of claim 1, including a further step of adding to the melt up to about 10 atomic percent of a metal selected from the group consisting of Mo, W, Nb, Zr, B, Va, Ta, Mn, Fe, Co or mixtures thereof.
 7. The method of claim 1, including the further step of adding to the melt up to about 5 atomic percent of a metal selected from the group consisting of Al, Ti or mixures thereof, and adding to the melt up to about 10 atomic percent of a metal selected from the group consisting of Mo, W, Ta, Nb, Va, Zr, B, Mn, Fe, Co or mixtures thereof.
 8. The method of claim 5, including the further steps of heating the Ni-Cr-Be type material containing the additive metal or mixtures of additive metals for a sufficient time to dissolve additive metal or mixtures of additive metals in solid solution, and strengthening the Ni-Cr-Be type material by holding the material below the solution temperature for a sufficient time to effect strengthening. 